Method and apparatus for lte radio link failure determiniation in drx mode

ABSTRACT

A method and apparatus for detecting radio link failure (RLF) in a wireless transmit receive unit (WTRU) includes the WTRU performing a series of radio link measurements during a discontinuous reception (DRX) on-duration, comparing each of the series of radio link measurements to a threshold, and determining that the series of radio link measurements indicates an out-of-synch condition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/693,898 filed on Dec. 4, 2012; which is a continuation of U.S. patentapplication Ser. No. 12/564,177 filed on Sep. 22, 2009, issued as U.S.Pat. No. 8,351,922 on Jan. 8, 2013; which claims the benefit of U.S.provisional applications Nos. 61/099,040, filed Sep. 22, 2008 and61/110,838 filed Nov. 3, 2008, the contents of which are incorporated byreference as if fully set forth herein.

FIELD OF INVENTION

This application is related to wireless communications.

BACKGROUND

The Third Generation Partnership Project (3GPP) has initiated the LongTerm Evolution (LTE) program to bring new technology, new networkarchitecture, new configurations and new applications and services towireless networks in order to provide improved spectral efficiency andfaster user experiences.

FIG. 1 shows an overview of an Evolved Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access Network(E-UTRAN) 100 in accordance with the prior art. As shown in FIG. 1,E-UTRAN 100 includes three eNodeBs (eNBs) 102, however, any number ofeNBs may be included in E-UTRAN 100. The eNBs 102 are interconnected byan X2 interface 108. The eNBs 102 are also connected by an S1 interface106 to the Evolved Packet Core (EPC) 104 that includes a MobilityManagement Entity (MME) 108 and a Serving Gateway (S-GW) 110.

FIG. 2 shows an LTE user-plane protocol stack 200 in accordance with theprior art. The protocol stack 200 is located in a wireless transmitreceive unit (WTRU) 210 and includes the packet data control protocol(PDCP) 202, the radio link control (RLC) 204, the medium access control(MAC) 206 and the physical layer (PHY) 208. The protocol stack 200 mayalso reside in an eNB (not shown).

FIG. 3 shows an LTE control plane protocol stack 300 of the WTRU 210 ofFIG. 2. The control plane protocol stack 300 includes the non-accessstratum (NAS) 302 and a radio resource control (RRC) 304. Also includedare the PDCP 306, RLC 308 and MAC 310, which together form the layer 2sublayer 312.

Malfunction and dysfunction of the radio link, that is, the link betweenthe WTRU and the eNB, may occur due to various reasons such asshielding, fading, interference or other mishaps due to mobility, forexample. Rapid detection of a radio link failure (RLF) is important inorder to initiate radio link or WTRU recovery procedures in a timelymanner. Typically, RLF detection comprises downlink signal measurementperformed by the physical layer entity combined with event filtering, sothe WTRU may determine a course of action to follow after the problem isdetected.

While performing downlink measurements, the physical layer (PHY) entity(Layer 1) of the WTRU may be configured to indicate a measurement resultof “out-of-sync” or “in-sync” to the RRC entity. The WTRU is configuredto count the number of out-of-sync results. The number of out-of-syncresults could be counted in a counter, such as counter N310, forexample. When the RRC entity counts a particular number of out-of-syncresults, the RRC entity within the WTRU is configured to start a timer.For example, the WTRU may start a timer designated as the T310 timer. Ifthe T310 timer expires prior to being stopped for another reason, theWTRU is configured to determine that an RLF has occurred.

FIG. 4 shows a discontinuous reception (DRX) cycle 400 in accordancewith the prior art. Connected state DRX was introduced for LTE compliantWTRUs and eNBs for power saving purposes, in particular to conserve thebattery of the WTRU. When a WTRU is in connected state DRX mode, it may“shut down” for period of time and use less power. As shown in FIG. 4,during a DRX cycle (402, 404, 406) a WTRU is able to transmit andreceive during an on-duration (408, 410, 412) and does not transmit orreceive during a sleep time (414, 416, 418). An eNB may be synchronizedwith the WTRU's DRX cycle, so that it does not send or expect to receivetransmissions while the WTRU is in a sleep time.

Three parameters may be used by the WTRU and eNB to define the WTRU'sDRX cycle. DRX on/off, DRX period and a non-DRX timer may be assignedvalues that network components can use to determine a WTRU's DRX cycle.

An LTE compliant WTRU may also be configured to operate in multiplestates, where each state defines how the WTRU may be functioning ingeneral terms. The RRC_connected state is one of the set of predefinedstates that a WTRU may function in. While in RRC_connected state, theWTRU may also be configured to operate in DRX mode.

When a WTRU is configured by the network to operate in DRX mode while inthe RRC_connected state, the WTRU may be configured not to performdownlink measurements for RLF during the sleep time portion of the DRXcycle. The WTRU may be configured to perform the downlink radio signalmeasurement with respect to RLF only during the DRX on-duration and theactive period.

SUMMARY

Disclosed is a method and apparatus for detecting a radio link failure(RLF) in a WTRU. This may include the WTRU performing a series of radiolink measurements during a discontinuous reception (DRX) on-duration,comparing each of the series of radio link measurements to a threshold,and determining that the series of radio link measurements indicates anout-of-synch condition. The WTRU may also start a timer and continue theseries of radio link measurements during a DRX sleep-time. The WTRU mayalso determine that the timer has expired and stop the series of radiolink measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1 shows an overview of an Evolved Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access Network(E-UTRAN) 100 in accordance with the prior art;

FIG. 2 shows an LTE user-plane protocol stack 200 in accordance with theprior art;

FIG. 3 shows an LTE control plane protocol stack 300 of the WTRU 210 ofFIG. 2;

FIG. 4 shows a DRX cycle for a WTRU in accordance with the prior art;

FIG. 5 shows an example wireless communication system including aplurality of WTRUs and an eNB;

FIG. 6 is a functional block diagram of a WTRU and the eNB of FIG. 2;

FIG. 7 shows a method to determine RLF in accordance with an embodiment;

FIG. 8 shows a method of RLF detection in accordance with anotherembodiment; and

FIG. 9 shows a method of RLF detection in accordance with yet anotherembodiment.

DETAILED DESCRIPTION

When referred to hereafter, the terminology “wireless transmit/receiveunit (WTRU)” includes but is not limited to a user equipment (UE), amobile station, a fixed or mobile subscriber unit, a pager, a cellulartelephone, a personal digital assistant (PDA), a computer, or any othertype of user device capable of operating in a wireless environment. Whenreferred to hereafter, the terminology “base station” includes but isnot limited to a Node-B, a site controller, an access point (AP), or anyother type of interfacing device capable of operating in a wirelessenvironment.

FIG. 5 shows a wireless communication system 500 including a pluralityof WTRUs 510, an eNB 520. As shown in FIG. 5, the WTRUs 510 are incommunication with the eNB 520, which could be in communication with oneanother as shown in FIG. 1. Although three WTRUs 510 and one eNB 520 areshown in FIG. 5, it should be noted that any combination of wireless andwired devices may be included in the wireless communication system 500.

FIG. 6 is a functional block diagram 600 of a WTRU 510 and the eNB 520of the wireless communication system 500 of FIG. 5. As shown in FIG. 6,the WTRU 510 is in communication with the eNB 520. The WTRU isconfigured to function in DRX mode or non-DRX mode. The WTRU may alsofunction in RRC_connected state or RRC-Idle state. The WTRU may beconfigured to perform methods for RLF determination in DRX mode andnon-DRX mode.

In addition to the components that may be found in a typical WTRU, theWTRU 510 includes a processor 615, a receiver 616, a transmitter 617,and an antenna 618. The processor 615 is configured to perform methodsfor RLF determination in RRC_connected state and DRX mode. The receiver616 and the transmitter 617 are in communication with the processor 615.The antenna 618 is in communication with both the receiver 616 and thetransmitter 617 to facilitate the transmission and reception of wirelessdata.

In addition to the components that may be found in a typical basestation, the eNB 520 includes a processor 625, a receiver 626, atransmitter 627, and an antenna 628. The processor 625 is configured toperform methods for RLF determination in RRC_connected state and DRXmode. The receiver 626 and the transmitter 627 are in communication withthe processor 625. The antenna 628 is in communication with both thereceiver 626 and the transmitter 627 to facilitate the transmission andreception of wireless data.

FIG. 7 shows a method to determine RLF 700 in accordance with anembodiment. Each DRX cycle (702,704,706) includes an on-duration(708,710, 712). Measurement (714-732) may be taken during eachon-duration (708,710, 712). Each measurement (714-732) is compared tothresholds Qin and Qout. Qin is a threshold for in synch operation, andis defined as a level at which downlink radio quality can besignificantly more reliably received than that at the out of syncthreshold (Qout). Qout is the out of synch threshold and is defined asthe level at which the downlink radio link cannot be reliably received.The RLF evaluation may be based on the number of Qin and Qout resultstaken by the PHY entity and transmitted to higher layer entities.

As shown in FIG. 7, during a first on-duration 708, a first measurement714 passed the in-synch (Qin) threshold while the second measurement 718and third measurement 720 did not reach the out-of synch (Qout)threshold. This results in an out-of-synch determination for the firstDRX cycle 702. In the second DRX cycle 704, during the secondon-duration 710, the first measurement 722, second measurement 724 andthird measurement 726 each resulted in an out-of-synch determination. Inthe third DRX cycle 706, during the third on-duration 712, the firstmeasurement 728, second measurement 730 and third measurement 732 eachresulted in an out-of-synch determination.

In FIG. 7, the WTRU is operating in DRX mode in RRC_connected state. RLFevaluation may be based on the measured status of a predetermined numberof consecutive DRX on-durations (N_(RLF-durations)), rather thanevaluating the measurements over a longer, continuous time period. Asshown in FIG. 7,

N_(RLF-durations) equals three (3), and the RLF analysis may depend onthe measured results of the first DRX cycle 702, which is out-of-synch,the second DRX cycle 704, which is also out-of-synch, and the third DRXcycle 706, which is also out of synch.

N_(RLF-durations) may be determined based on a number of criteria. Forexample, N_(RLF-durations) may be a fixed number that is preconfiguredin the WTRU or signaled by the network. N_(RLF-durations) may also bethe value of a counter, such as N313 for example, or the value of thecounter divided by an integer M, where M is a timer value divided byon-duration. N_(RLF-durations) may also be a function of the length of atimer, such at timer T310, the length of the signaled on-duration timeas computed by the WTRU, such as N_(RLF-durations)=(T310/on-durationperiod), or a function of the DRX cycle length, for example, the mediumaccess control (MAC) DRX cycle, the long DRX-cycle and the shortDRX-cycle, optionally including the length of the on-duration timer.

The on-duration timer length may be configured by the network andtransmitted to the WTRU. The on-duration timer length may also becomputed by the WTRU. The length of the DRX cycle and the number ofconsecutive on-duration measurements used to compute RLF may beinversely proportional. Optionally N_(RLF-durations) may equal(configured DRX-length)/(shortest configurable DRX-length). As anotheroption, N_(RLF-durations) may equal the DRX-length/W, where W is anetwork configured or preconfigured integer.

Again referring to FIG. 7, at each on-duration period of each DRX cycle(708,710,712) higher layer entities in the WTRU may receive the measuredRLF values from the (PHY) entity. The PHY entity may evaluate anddetermine, on a per DRX cycle basis, if the WTRU is in-synch orout-of-synch with the network. The PHY entity may then send a message tothe higher layers, such as the MAC layer entity, radio resource control(RRC) layer entity, or radio link control (RLC) layer entity, thatspecifies “in-synch” or “out-of-sync”, without passing the measurementdata. The PHY entity will transmit the in-synch or out-of-synchcondition by determining if, over the measurement period, there are amajority of in-synch measurements or out-of-synch measurements. If thereis the same number of each measurement, the last returned measurement isused to determine whether the out of synch or in synch condition ispassed to the higher layers.

Referring again to FIG. 7, the PHY entity may determine that, for thefirst DRX cycle 702, an out-of-synch condition should be transmitted tothe higher layers, as there are two out-of-synch measurements (718, 720)and only one in-synch measurement (714). Similarly, for the second DRXcycle 704 and the third DRX cycle 706, the PHY entity would transmit anindication of an out-of-synch condition, as all the measurements in eachcycle are out-of-synch.

The WTRU may be configured to determine that an RLF condition existsonly when all N_(RLF-durations) on-durations, or the DRX cyclescontaining the on-duration, measure as out-of-sync. The RRC may beconfigured to treat the WTRU in DRX mode in the same way as if a timer,such as the T310 timer, had expired and the WTRU was in non-DRX mode.

When the WTRU is in DRX mode, RLF measurements may be discontinuous, dueto the nature of DRX operation. For a WTRU in DRX mode, the PHY layerentity may be configured to implicitly use different Qout and Qinthresholds than when the WTRU in non-DRX mode. For example, if the WTRUis in DRX mode, the PHY layer entity may apply an offset to thethreshold values that the WTRU may use in non-DRX mode. This offset maylower the threshold values for Qout and Qin. A lower Qout thresholdmeans that a lower measured value would be required to declare anout-of-synch condition in DRX mode than in non-DRX mode. A lower Qinthreshold means it is easier in DRX mode to meet the in-synchmeasurement value than in non-DRX mode. Therefore, in DRX mode, therequirements for reaching an in-synch condition are more relaxed that innon-DRX mode.

The WTRU may make another adjustment during DRX mode in itsdetermination of RLF conditions. During DRX mode, the PHY entity may beconfigured to reduce its filtering time from the non-DRX mode time tothe length of the on-duration time, or shorter, if the on-duration timeis shorter than the non-DRX filtering timer. For example, if the non-DRXfiltering time is 200 ms, in DRX mode the WTRU may use a time shorterthan 200 ms. The adjustment may be made by the WTRU based on an offset,by a fraction or value that is signaled from the network or predefined.The shorter filtering time in DRX mode should be sufficient for the WTRUto take measurements and check the downlink radio link quality of theserving cell.

Alternatively, while in DRX mode, the PHY entity may be configured tomonitor every radio frame to check and measure downlink radio qualityduring the on-duration intervals only, as measured over a number (m) ofDRX periods against thresholds (Qout and Qin). The number m is aparameter signaled by the network or derived by the WTRU based on thechannel conditions. In the event that the on-duration is less than aframe, the WTRU may be configured to evaluate link quality over theon-duration interval only.

In another embodiment, during each DRX cycle, measurements are takenduring the on-duration. The WTRU may continue measuring past eachon-duration boundary and into the sleep time portion of the DRX cycle.This may occur when a single or small preconfigured number ofconsecutive Qout measurements are detected. The WTRU may also continuemeasuring during the sleep time if an average value of all measurementstaken during the on-duration is below the out-of-sync threshold, untileither a set number of consecutive Qin is measured or the RLF isdeclared The DRX logic of data reception may remain unchanged.

The WTRU may be configured such that once the set number of consecutiveQout is detected, the RLF measurement operation continues into the sleeptime as if the WTRU is operating in the non-DRX mode. Therefore thenon-DRX RLF detection criteria, for example, using a network configuredtime such as the RLF recovery timer T310 to gauge the RLF determination,may be used, even though the WTRU is in DRX mode. If, however, before orwhen the length of evaluation period is completed, for example, 200 ms,and if the in-synch condition is met, for example, a set number ofconsecutive Qin are detected, the RLF measurement may be stopped for theinactive portion of time of the DRX cycle.

FIG. 8 shows a method of RLF detection 800 in accordance with anotherembodiment. In the first DRX cycle 802, three measurements (804, 806,808) are taken during the first on-duration 810. During the second DRXcycle, three measurements (814, 816, 818) are taken during the secondon-duration 820 and three measurements (822, 824, 826) are taken duringthe sleep time 828 of the second DRX cycle 812. This may occur due tothe detection of three consecutive out-of-synch measurements. In thethird DRX cycle 830, during the third on-duration 832, threemeasurements (834, 836, 838) are taken.

In an alternative embodiment, when the WTRU is in DRX mode, and aparticular number of out-of-synch measurements are detected, the WTRUmay start a timer for recovery. The WTRU may also continue to make RLFmeasurements during the sleep time of the DRX cycle. The duration of therecovery measurements may be proportional to the number of in-synchmeasurements required for radio link recovery or to the duration of asingle in-synch measurement. When a predetermined number of Qoutmeasurements are detected, the RLF measurements may be continued into arecovery period, which is a period of time used by the WTRU to determineif the measured results are getting better or remains out-of-synch. If,during the recovery period, the measurements are determined to bein-synch, then the radio link has improved. If not, the WTRU maydetermine that an RLF has occurred.

The measurements may alternatively continue until the timer expires, asthe expiry of the timer indicates a RLF. For example, the numbermeasurements that occur during the sleep time, referred to as “recoverymeasurements” (Nr) may each have a duration, Dr, with an interval of Trbetween measurements, such that Nr×Dr=K×(counter value)×Tinsynch(Equation 1) where K is a predefined constant and Tinsynch is a durationof an in-synch measurement. Furthermore, Tr=timer value/Nr. If Tr isless that or equal to Dr, the recovery measurements occur continuously.The timer value is the allowed time span for a higher layer entityfilter to receive a number (counter value) of consecutive in-synchsignals from the PHY entity in order to cancel the radio linkout-of-sync state before a RLF is determined.

The WTRU may use an out-of-synch counter while in DRX mode. The value ofthe out-of-synch counter may be dependent upon the RLF timer used in DRXmode and the number of in-synch or out-of-synch indications sent by thePHY entity to the upper layer entities per DRX cycle. For example, theDRX mode out-of-synch counter may equal ((DRX mode timer value)/(DRXcycle length))×(number of indications (Nsigns−L1). The DRX mode timervalue may equal the non-DRX mode timer value plus the DRX cycle length.

Nsigns−L1 may be determined by the value of the on-duration timer andthe DRX cycle length. For WTRUs with a short DRX cycle length, Nsigns−L1may equal one (1). For WTRUs with a long DRX cycle length, Nsigns−L1 mayequal the long cycle length divided by the short cycle length. For DRXmode, the in-synch counter may be based on the out-of-synch timer plusor minus a constant.

If the WTRU reads reference signal (RS) quality or physical downlinkcontrol channel (PDCCH) block error rate (BLER) during the sleepduration, and compares that to an RS quality or PDCCH BLER reading takenbefore the DRX sleep duration, the WTRU may determine radio link qualityand the DRX cycle may have no impact on RLF detection time.

The WTRU may monitor the PDCCH, including decoding the cell radionetwork temporary identifier (C-RNTI), the system information radionetwork temporary identifier (SI-RNTI), the paging radio networktemporary identifier (P-RNTI) and all other relevant RNTI during theactive period of the DRX cycle. The monitoring may be activated if theWTRU detects an out-of-synch measurement in a particular DRX cycle. Ifthe RNTI is decoded successfully, then the PHY entity may transmit anin-synch indication to the higher layer entities. If the decoding of theRNTI fails, due to CRC failure, for example, for a certain number ofattempts, then the PHY entity may transmit an out-of-synch indication tothe higher layers.

FIG. 9 shows a method of RLF detection 900 in accordance with yetanother embodiment. At step 902, a radio link status counter is set tothe value of a parameter such as RADIO_LINK_TIMEOUT. The parameter maybe predefined and is related to the number of out-of-synch signalsrequired for the WTRU to determine RLF. At step 904, a higher layerentity filter receives a radio link monitor indicator and at step 906the radio link status counter is updated according to the values inTABLE 1. At step 908, which occurs at each DRX cycle, the WTRU reads theradio link status counter. If the radio link status counter is not equalto zero, the process continues to step 902. If the radio link statuscounter does equal zero, then at step 910, either the WTRU can determinethat RLF has occurred or the WTRU may start a recover timer.

TABLE 1 Update value to RL Monitoring DPCCH Monitoring the “radio linkNo Sign Sign status counter” 1 InSync Success +2 2 InSync Failure 0 3InSync No-sign +1 4 OutSync Success 0 5 OutSync Failure −2 6 OutSyncNo-sign −1

TABLE 1 may be modified by giving more or less weight to a measurementor giving more or less weight to the DPCCH monitoring sign. Furthermore,in order for the WTRU to obtain a consistent estimated PDCCH BLER, thePDCCH BLER may be based on the shortest PDCCH format or the largestaggregation level.

For radio link recovery, i.e. when (N<N_(RLF-durations)) out-of-syncinstances are reported, the WTRU may require N_(RLF-durations) or anumber slightly smaller than N_(RLF-durations), of in-sync instances torecover the link.

Although features and elements are described above in particularcombinations, each feature or element can be used alone without theother features and elements or in various combinations with or withoutother features and elements. The methods or flow charts provided hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable storage medium for execution by ageneral purpose computer or a processor. Examples of computer-readablestorage mediums include a read only memory (ROM), a random access memory(RAM), a register, cache memory, semiconductor memory devices, magneticmedia such as internal hard disks and removable disks, magneto-opticalmedia, and optical media such as CD-ROM disks, and digital versatiledisks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) or Ultra Wide Band (UWB)module.

What is claimed is:
 1. A method of radio link failure (RLF) in awireless transmit receive unit (WTRU), the method comprising: performinga series of radio link measurements during discontinuous reception (DRX)mode; detecting a predetermined number of consecutive out-of-synchconditions based on the series of radio link measurements; starting atimer upon detecting the predetermined number of consecutiveout-of-synch conditions; and while operating in DRX mode, continuing theseries of radio link measurements using an evaluation periodcorresponding to non-DRX mode.